Kids taking part in a study of how much they exercised tricked researchers by attaching their pedometers to dogs.
About 200 kids in east London were given the special tags which monitored how many steps they walked or ran.
But researchers at Mile End Centre for Sports and Exercise Medicine was surprised by how active some obese children seemed to be.
Professor Nicola Maffulli said: "Then we realised they were attaching the pedometers to their dogs' collars."
Once the results had been adjusted to take into account the cheats, it was found kids in the area weren't very active.
They walked 12,620 steps a day, below the recommended level of 15,000 steps.